The mode of failure of structures designed for specific end uses such as gun barrels can be determined by basic mechanisms. One such mechanism is the rate at which heat can be transferred from a surface which receives the heat through the structure to a surface which can dissipate the heat. For example, in a gun barrel the heat is received by the barrel at the barrel interior due to the burning and heat of burning of the propellant material. In addition, frictional forces of the projectile moving along and against the surface of the interior of the barrel can generate heat at the immediate surface contacted by the projectile. Where the amount of heat that can be removed from the barrel through the normal conduction mechanism is limited, this places a limit also on the application which can be made of the gun. If temperatures become excessive, the gun barrel may fail either locally at the inner surface of the gun barrel by localized melting or metal deformation at high temperature or the physical properties of the overall structure of the barrel may deteriorate resulting in a rupture.
Another mode of failure is the simple mechanical failure to contain the mechanical forces which are applied on the gun barrel. For example, as a propellant is ignited and burns it generates not only heat but also very high pressure and this pressure must be mechanically contained by the barrel. Also, where the projectile leaves its cartridge and starts down the barrel the rifling on the barrel mechanically applies a torsional force to the projectile to give it spin necessary to aid it in its accurate flight to a destination or target. Where the mechanical force needed to initiate rotation of the projectile is excessive, mechanical failure of the barrel can occur at the location adjacent the chamber where the barrel rifling starts.
The heat generated at the bore of a gun barrel can build up very rapidly in spite of the fact that heat can be transferred through the wall of the barrel to the barrel exterior because of the higher rate at which heat can be produced at the bore compared to the rate at which the produced heat can be carried by heat conduction through the thickness of the barrel wall. For a barrel wall of lower conductivity, when long bursts of firing occur, or when the heat produced by the gases is relatively high, this heat production is concentrated at the bore surface and cannot be conducted from the bore rapidly enough because of the limitations in conductivity of heat through the material of the barrel wall.
There is a heat sink effect in the thickness of the barrel but this heat sink is available only until the temperature of the barrel itself is raised by production of heat within the bore in excess of the quantity of heat which can be conducted through the wall thickness based on the characteristics of the material of the wall itself.
There is thus a need for a gun barrel with improved thermal conductivity that at the same time has great yield strength, that is, resistance to permanent deformation and fracturing.
Another reason for improved thermal conductivity is to improve the accuracy of the gun barrel. Accuracy is a function of barrel stiffness and resistance to thermal heating. That is, it is known that after firing a number of bullets through a conventional steel gun barrel, the barrel will grow quite hot. This may cause the barrel to warp, reducing the accuracy of firing. Improved stiffness of a gun barrel tends to reduce the harmonic oscillations caused by the firing of the bullet within the barrel, which also increases accuracy of firing.
There is thus a need for a gun barrel that, in addition to improved thermal conductivity and greater yield strength, has greater stiffness as measured by Young""s modulus.
Finally, the material selected for the gun barrel should be as light as possible consistent with the above requirements, in order to make the weapon less heavy to carry.
Increasing the yield strength of a metal by incorporation of hard particles is known to occur. In theory, yield strength is increased by inhibiting the dislocation mobility of the native metal. Dislocations are basically points or lines or various shapes of missing atoms in a crystal structure. When a load or force is applied to the structure, those lines of missing atoms tend to move from inside the structure to the outside of the material. The more readily that such dislocations can move from the inside to the outside of the material, the lower the yield strength. Incorporation of small, hard particles into the metal puts obstacles in the way of the moving dislocations, tending to prevent such dislocations from reaching the surface of the metal, where permanent deformation and fracture can occur.
The yield strength of aluminum can be increased by the incorporation of ceramic particles such as aluminum oxide and silicon carbide. However, these materials typically have lower thermal conductivity than aluminum, and therefore are not suitable strengthening materials where the end product must have high thermal conductivity. In contrast, diamond has one of the highest thermal conductivities known: about five times that of copper. Therefore, diamond particles ought to increase both the yield strength and the thermal conductivity of a gun barrel.
There is a need for a composite gun barrel that has the same stiffness of steel at room temperature but maintains this stiffness upon heating and also transfers heat more readily and is more resistant to permanent deformation and fracture than steel at higher temperatures. The material should also have a lower coefficient of thermal expansion, so that it maintains its original shape as much as possible at higher temperatures.
While this material is well suited to gun barrels, it would also have application to any technology which requires high strength, high thermal conductivity, high stiffness, and a low coefficient of thermal expansion. Such applications may include rocket nozzles, cookware, heat sinks, electronics packaging, and automotive components such as brake disks, brake drums, transmission components, and engine components.
The objective of the present invention is an aluminum-diamond composite and a method of manufacture for the same.
The basic approach used to produce a structural composite material with high thermal conductivity is to combine pure aluminum or an aluminum alloy with industrial grade diamond particles (a common abrasive). The particles selected are preferably Type IB monocrystalline diamonds with a maximum size of 2.0 xcexcm. Aluminum and diamond have nearly the same density, but the real advantage for this material comes from the fact that diamond is one of the stiffest, strongest, and most thermally conductive materials known. Intrinsic material properties such as Young""s modulus and thermal conductivity can be estimated for composites based on the xe2x80x9crule of mixtures.xe2x80x9d In other words, the properties of the composite are the sum of the properties of the constituent materials times their volume fraction.